GB691865A - Improvements in or relating to relay station apparatus for use in telecommunication systems - Google Patents

Abstract

691,865. Automatic frequency control systems. GENERAL ELECTRIC CO., Ltd., and STENNING, L. C. May 21, 1951 [May 19, 1950], No. 12624/50. Class 38 (iv). [Also in Group XL (c)] A relay station for frequency or phase modulated signals comprises an oscillation generator, the operating frequency of which determines the frequency of the transmitted oscillations, and means to supply an electric signal which carries the received modulation to the oscillation generator so as to control the operating frequency thereof in dependence upon the instantaneous difference in frequency or phase of two oscillations, one of which is the received oscillations or has a frequency dependent upon the frequency of the received oscillations and the other is the transmitted oscillations or has a frequency dependent upon the frequency of the transmitted oscillations whereby the said difference is maintained substantially at a predetermined value. As shown, the received oscillations are fed to a frequency changer 11 together with locally-generated oscillations, and the difference frequency is passed through an amplifier 12 to a discriminator 13. The output from the discriminator includes the modulation signal and frequency modulates the oscillator 14 of the transmitter, the output from the oscillator being passed through a frequency multiplier 15 and amplifier 16 to the transmitting aerial. A portion of the output from the frequency multiplier 15 is mixed in a frequency changer 18 with oscillations from a crystal-controlled oscillator 19, to provide the locally-generated oscillations applied to the frequency changer 11. The discriminator 13 is tuned to the frequency of the constant frequency oscillations from oscillator 19 plus or minus the required difference in the mean frequencies of the received and transmitted oscillations, and provides an output consisting of a unidirectional component, together with the modulation component, the unidirectional component providing the correcting signal for the oscillator 14. A phasecorrecting network may be included in the control signal path between the discriminator 13 and oscillator 14 to reduce any tendency to loop instability. Reference has been directed by the Comptroller to Specification 653,288.